After the second world war, John Piper, Edward Bawden and other artists were commissioned to spruce up Lyons teashops. Michael Prodger reflects on a halcyon time of 'art for all' – and a project that could never happen today

From Lucian Freud's stolen portrait of his friend Francis Bacon to a homemade dadaist grotto bombed in the second world war, here are the extraordinary tales of some of the world's most legendary lost artworks